Method of forming semiconductor device having stacked transistors

ABSTRACT

There is provided a method of forming a semiconductor device having stacked transistors. When forming a contact hole for connecting the stacked transistors to each other, ohmic layers on the bottom and the sidewall of the common contact hole are separately formed. As a result, the respective ohmic layers are optimally formed to meet requirements or conditions. Accordingly, the contact resistance of the common contact may be minimized so that it is possible to enhance the speed of the semiconductor device.

FIELD OF THE INVENTION

The present invention relates to a method of forming a semiconductordevice, and more particularly, to a method of forming a semiconductordevice having stacked transistors.

BACKGROUND OF THE INVENTION

As a semiconductor device is highly integrated, a lot of research forstructures in which a plurality of transistors are multi-stacked withina limited area of a semiconductor substrate has been conducted. A staticrandom access memory (SRAM) device, for example, is one of thesemiconductor devices employing a stacked structure of transistors.

FIG. 1 is an equivalent circuit diagram of a conventional inverter.

Referring to FIG. 1, a gate electrode of a first transistor TR1, e.g.,an n-channel metal oxide semiconductor (NMOS) transistor, and a gateelectrode of a second transistor TR2, e.g., a p-channel MOS (PMOS)transistor, are commonly connected to an input signal V_(in) line. Asource of the second transistor TR2 is connected to a power voltageV_(DD) line, and drains C of the second transistor TR2 and the firsttransistor TR1 are connected to an output line V_(out). A source of thefirst transistor TR1 is connected to a ground voltage V_(SS) line. Thedevice having the above circuit structure may operate as a typicalinverter.

The inverter of FIG. 1 may be formed by arranging the first and secondtransistors TR1 and TR2, of which conductive types are different fromeach other, on the same plane. However, for high integration of thedevice, it may be better to configure the inverter as the stackedstructure where the second transistor TR2 is stacked on the firsttransistor TR1.

FIG. 2 is an equivalent circuit diagram of a conventional full CMOS(Complementary Metal-Oxide Semiconductor) SRAM device.

Referring to FIG. 2, the conventional full CMOS SRAM device isconfigured with two inverters and two transfer transistors TR3 and TR6.The inverters are configured with driving transistors TR1 and TR4, e.g.,NMOS transistors, and load transistors TR2 and TR5, e.g., PMOStransistors, wherein the two inverters are cross-coupled to each other.A source of each load transistor TR2 and TR5 is connected to a powervoltage V_(DD) line, and a source of each driving transistor TR1 and TR4is connected to a ground voltage V_(SS) line. Gate electrodes of thetransfer transistors TR3 and TR6 are connected to a word line, and oneof source/drain thereof is connected to a bit line BL and BL. The otherone of the source/drain of each transfer transistor TR3 and TR6, a drainof each driving transistor TR1 and TR4, and a drain of each loadtransistor TR2 and TR5 are commonly connected to each of commonterminals/contacts C1 and C2. In addition, the common terminal C1 isconnected to the gate electrode of the driving transistor TR4 and thegate electrode of the load transistor TR5. Likewise, the common terminalC2 is also connected to the gate electrode of the driving transistor TR1and the gate electrode of the load transistor TR2.

Although the SRAM device may be formed by arranging the six transistorsTR1 to TR6 on the same plane, it may be formed by arranging the drivingtransistors TR1 and TR4 at a lowermost portion, the load transistors TR2and TR5 on the driving transistors TR1 and TR4, and the transfertransistors TR3 and TR6 on the load transistors TR2 and TR5, and thus,it is possible to enhance the integration of the device by employingthis stacked structure.

In the semiconductor device having the structure of the stackedtransistors, the common contact such as the common terminals C1 and C2may be used to connect the stacked transistors to each other. A methodof forming the common contact in the semiconductor device having thestacked transistors will be set forth herebelow.

A plurality of interlayer insulating layers stacked on a semiconductorsubstrate, and a semiconductor single crystalline layer interposedtherebetween, are patterned into a predetermined configuration tothereby form a common contact hole, wherein source/drain regions of atransistor are disposed at a predetermined position of the semiconductorsingle crystalline layer. Thereafter, a metal layer is conformallyformed on the resultant structure and an annealing process is performedso as to form an ohmic layer, e.g., a metal silicide layer, on thesemiconductor substrate exposed by the contact hole, and sidewalls ofthe semiconductor single crystalline layer exposed by the contact hole.Afterwards, a conductive layer is formed over the resultant structure tofill the common contact hole, to thereby form a common contact plug.

The contact resistance of the common contact may be varied with thestate of the ohmic layer disposed on the bottom and the sidewall of thecommon contact. For example, an area of the exposed semiconductorsubstrate under the bottom of the common contact is different from anarea of the exposed semiconductor single crystalline layer at thesidewall of the common contact. There is also difference between thethicknesses of the respective ohmic layers on the bottom and thesidewall of the common contact, in order to optimally drive the device.In detail, if the metal layer with a predetermined thickness is formedon the bottom of the common contact to enable the ohmic layer on thebottom to have an optimized thickness, the metal layer on the sidewallof the common contact may be formed thinly in comparison with the metallayer on the bottom. Otherwise, as illustrated in FIG. 3, a void mayform between the semiconductor single crystalline layer and the ohmiclayer on the sidewall of the common contact so that the resistance maybe increased. On the contrary, if the metal layer with a certainthickness is formed on the sidewall of the common contact to enable theohmic layer on the sidewall to have an optimized thickness, the ohmiclayer on the bottom of the common contact may be formed thickly incomparison with the ohmic layer on the sidewall. Resultingly, asillustrated in FIG. 4, a spike is formed so that a leakage current mayoccur.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide methods of forming asemiconductor device having stacked transistors capable of optimizingohmic layers which are formed on both a bottom and a sidewall of acommon contact.

In the method of forming the semiconductor device having the stackedtransistors according to embodiments of the present invention, ohmiclayers on the bottom and the sidewall of the common contact hole areseparately formed.

Embodiments of the present invention provide methods of forming asemiconductor device, the method including: forming a plurality ofinterlayer insulating layers and a semiconductor single crystallinelayer interposed therebetween, over a semiconductor substrate; forming acommon contact hole to expose the semiconductor substrate by patterningthe plurality of the interlayer insulating layers and the semiconductorsingle crystalline layer interposed therebetween; forming a first ohmiclayer to cover a sidewall of the semiconductor single crystalline layerwhich is exposed by the common contact hole; forming a second ohmiclayer to cover the semiconductor substrate which is exposed by thecommon contact hole; and forming a common contact plug to fill thecommon contact hole.

In some embodiments, the methods further include, before the forming ofthe first ohmic layer, forming a blocking layer to expose the sidewallof the semiconductor single crystalline layer and cover the bottom ofthe common contact hole, wherein the blocking layer is removed afterforming the first ohmic layer. The blocking layer may be formed of amaterial which may be selectively removed in a following process withoutany reaction with the underlying semiconductor substrate using thecharacteristic that a step coverage is poor at sidewalls. For example,the blocking layer is formed using physical vapor deposition (PVD) orsputtering. The blocking layer is formed of titanium nitride layer.

In other embodiments of the present invention, a method of forming asemiconductor device includes: forming a lower transistor and a lowerinterlayer insulating layer covering the lower transistor, over thesemiconductor substrate; forming an upper transistor and an upperinterlayer insulating layer covering the upper transistor, on the lowerinterlayer insulating layer; forming a common contact hole to exposesource/drain regions of the lower transistor by patterning the upperinterlayer insulating layer, source/drain regions of the uppertransistor, and the lower interlayer insulating layer; forming ablocking layer to cover the bottom of the common contact hole but toexpose the source/drain regions of the upper transistor; forming a firstohmic layer on the source/drain regions of the upper transistor exposedby the blocking layer; exposing the source/drain regions of the lowertransistor by removing the blocking layer disposed on the bottom of thecommon contact hole; forming a second ohmic layer on the exposedsource/drain regions of the lower transistor; and forming a commoncontact plug to fill the common contact hole with a conductive layer.

In further embodiments, before the forming of the common contact plug, abarrier metal layer is conformally formed. Each of the first and secondohmic layers is formed of a metal silicide, the metal being at least oneselected from the group consisting of titanium, cobalt, nickel, andtungsten.

In yet further embodiments, a method further includes, before theforming the upper transistor and the upper interlayer insulating layer,forming an intermediate transistor and an intermediate interlayerinsulating layer covering the intermediate transistor, on the lowerinterlayer insulating layer. In this case, the intermediate interlayerinsulating layer and source/drain regions of the intermediate transistorare patterned when forming the common contact hole.

In some embodiments, the common contact hole exposes at least one gateelectrode of the lower transistor and the intermediate transistor, andthe first ohmic layer is formed on the sidewall of the exposed gateelectrode.

In other embodiments of the present invention, a method of forming asemiconductor device, includes: forming a first transistor including afirst gate pattern and a first source/drain region on both sides of thefirst gate pattern, over a semiconductor substrate, wherein the firstsource/drain region is disposed in the semiconductor substrate; forminga first interlayer insulating layer to cover the first transistor;forming a first epitaxial contact plug in contact with the firstsource/drain region through the first interlayer insulating layer;forming a first semiconductor single crystalline layer on the firstinterlayer insulating layer, wherein the first semiconductor singlecrystalline layer is in contact with the first epitaxial contact plug;forming a second transistor including a second gate pattern and a secondsource/drain region on both sides of the second gate pattern, over thefirst semiconductor single crystalline layer, wherein the secondsource/drain region is disposed in the first semiconductor singlecrystalline layer; forming a second interlayer insulating layer to coverthe second transistor; forming a common contact hole to expose thesemiconductor substrate by patterning the second interlayer insulatinglayer, the first semiconductor single crystalline layer, and the firstepitaxial contact plug; forming a blocking layer to cover the bottom ofthe common contact hole but to expose the sidewall of the firstsemiconductor single crystalline layer; forming a first ohmic layer tocover the sidewall of the first semiconductor single crystalline layer;removing the blocking layer; forming a second ohmic layer on thesemiconductor substrate on the bottom of the common contact hole; andforming a common contact plug to fill the common contact hole.

In further embodiments, a method further includes, before the forming ofthe common contact hole: forming a second epitaxial contact plug incontact with the first semiconductor single crystalline layer throughthe second interlayer insulating layer; forming a second semiconductorsingle crystalline layer on the second interlayer insulating layer,wherein the second semiconductor single crystalline layer is in contactwith the second epitaxial contact plug; forming a third transistorincluding a third gate pattern and a third source/drain region on bothsides of the third gate pattern, over the second semiconductor singlecrystalline layer, wherein the third source/drain region is disposed inthe second semiconductor single crystalline layer; and forming a thirdinterlayer insulating layer to cover the third transistor. Herein, thethird interlayer insulating layer and the second semiconductor singlecrystalline layer are also patterned when forming the common contacthole, and the first ohmic layer is also formed on the sidewall of thesecond semiconductor single crystalline layer.

In yet further embodiments, the first and second semiconductor singlecrystalline layers are formed by forming an amorphous semiconductorlayer on the semiconductor substrate where the second epitaxial contactplug is formed; and transforming an amorphous structure of the amorphoussemiconductor layer into a single crystalline structure by performingannealing process. In addition, the first and second semiconductorsingle crystalline layers are formed by epitaxially growing an epitaxiallayer from the first and second epitaxial contact plugs, respectively,through an epitaxial growth.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is an equivalent circuit diagram of a conventional inverter;

FIG. 2 is an equivalent circuit diagram of a conventional fullcomplementary metal oxide semiconductor (CMOS) static random accessmemory (SRAM) device;

FIGS. 3 and 4 are scanning electron microscope (SEM) photographs showinga portion of a semiconductor device according to the conventional methodof forming the semiconductor device having stacked transistors;

FIGS. 5 to 12 are cross-sectional views illustrating a method of formingan inverter according to embodiments of the present invention; and

FIGS. 13 to 17 are cross-sectional views illustrating a method offorming an SRAM device according to further embodiments of the presentinvention.

DETAILED DESCRIPTION

The present invention now is described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

Like numbers refer to like elements throughout. In the figures, thethickness of certain lines, layers, components, elements or features maybe exaggerated for clarity. All publications, patent applications,patents, and other references mentioned herein are incorporated hereinby reference in their entireties.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. As used herein, phrases such as “between X and Y” and“between about X and Y” should be interpreted to include X and Y. Asused herein, phrases such as “between about X and Y” mean “between aboutX and about Y.” As used herein, phrases such as “from about X to Y” mean“from about X to about Y.”

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the specification andrelevant art and should not be interpreted in an idealized or overlyformal sense unless expressly so defined herein. Well-known functions orconstructions may not be described in detail for brevity and/or clarity.

It will be understood that when an element is referred to as being “on”,“attached” to, “connected” to, “coupled” with, “contacting”, etc.,another element, it can be directly on, attached to, connected to,coupled with or contacting the other element or intervening elements mayalso be present. In contrast, when an element is referred to as being,for example, “directly on”, “directly attached” to, “directly connected”to, “directly coupled” with or “directly contacting” another element,there are no intervening elements present. It will also be appreciatedby those of skill in the art that references to a structure or featurethat is disposed “adjacent” another feature may have portions thatoverlap or underlie the adjacent feature.

Spatially relative terms, such as “under”, “below”, “lower”, “over”,“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is inverted, elements described as “under” or “beneath” otherelements or features would then be oriented “over” the other elements orfeatures. Thus, the exemplary term “under” can encompass both anorientation of “over” and “under”. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly. Similarly, the terms“upwardly”, “downwardly”, “vertical”, “horizontal” and the like are usedherein for the purpose of explanation only unless specifically indicatedotherwise.

It will be understood that, although the terms “first”, “second”, etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a “first” element, component, region, layer or section discussed belowcould also be termed a “second” element, component, region, layer orsection without departing from the teachings of the present invention.The sequence of operations (or steps) is not limited to the orderpresented in the claims or figures unless specifically indicatedotherwise.

FIGS. 5 to 12 are cross-sectional views illustrating a method of formingan inverter according to embodiments of the present invention.

Referring to FIG. 5, a first isolation layer 3 is formed in asemiconductor substrate 1 using, for example, a typical shallow trenchisolation (STI) process or the like so as to define an active region. Athermal oxidation process is performed over the semiconductor substrate1 to form a first gate oxide layer 5 on the active region. Upon theentire surface of the semiconductor substrate 1, a first gate electrodelayer 7 and a first capping layer 9 are stacked, and they are patternedto thereby form a first gate pattern 11. The first gate electrode layer7 may be formed, for example, of at least one of polysilicon doped withimpurities, tungsten, tungsten silicide, and tungsten nitride. An ionimplantation process is performed using the first gate pattern 11 as anion implantation mask to form first impurity implantation regions 15 sand 15 d. The first impurity implantation regions may comprise a firstsource region 15 s and a first drain region 15 d. A first spacer 13 isformed covering sidewalls of the first gate pattern 11, to therebycomplete a first transistor. Although not shown in the drawings, aheavily doped region may be formed using the first spacer 13 and thefirst gate pattern 11. A first interlayer insulating layer 17 is formedto cover the first transistor.

Referring to FIG. 6, the first interlayer insulating layer 17 ispatterned to form a contact hole 19 which exposes the first drain region15 d. An epitaxial contact plug 21 is formed in the contact hole 19 bygrowing an epitaxial layer from a semiconductor single crystallinestructure of the exposed first drain region 15 d through epitaxialgrowth. A chemical mechanical polishing (CMP) process may be performedfor planarizing the top surface of the epitaxial contact plug 21.

Referring to FIG. 7, a semiconductor single crystalline layer 23 isformed on the entire surface of the semiconductor substrate 1. Thesemiconductor single crystalline layer 23, for instance, may be formedusing solid phase epitaxial growth (SPEG) process which transforms anamorphous structure of an amorphous polysilicon layer (not shown) into asingle crystalline silicon structure by annealing the amorphouspolysilicon layer formed over the entire surface of the semiconductorsubstrate 1. Herein, the top surface of the epitaxial contact plug 21may act as a single crystalline seed layer. Alternatively, thesemiconductor single crystalline layer 23 may be formed by epitaxiallygrowing an epitaxial layer from the epitaxial contact plug 21 usingselective epitaxial growth (SEG) process. In this case, a CMP processmay be additionally performed for planarizing a top surface thereof. Asecond isolation layer 25 is formed on the semiconductor singlecrystalline layer 23 using, for example, a general STI process. Althoughit is illustrated in FIG. 7 as if the second isolation layer 25 seems tobe in contact with the first interlayer insulating layer 17, the secondisolation layer 25 need not be in contact with the first interlayerinsulating layer 17.

Referring to FIG. 8, upon the semiconductor single crystalline layer 23,a second gate pattern 33 and a second spacer 35 covering sidewalls ofthe second gate pattern 33 are formed. The second gate pattern 33 isprovided with a second gate oxide layer 27, a second gate electrode 29and a second capping layer 31. An ion implantation process is performedto form a second source region 36 s and a second drain region 36 d, tothereby complete a second transistor. Thereafter, a second interlayerinsulating layer 37 is formed to cover the second transistor.

Referring to FIG. 9, the second interlayer insulating layer 37, thesemiconductor single crystalline layer 23 and the epitaxial contact plug21 are patterned in sequence to form a common contact hole 39 whichexposes the first drain region 15 d. The common contact hole 39 may beoverlapped with the contact hole 19, but it may be formed such that itis not overlapped with the contact hole 19. In case that the commoncontact hole 39 is not overlapped with the contact hole 19, the commoncontact hole 39 is formed by patterning the second interlayer insulatinglayer 37, the semiconductor single crystalline layer 23 and the firstinterlayer insulating layer 17 in sequence.

Referring to FIG. 10, a blocking layer 41 is formed on the entiresurface of the semiconductor substrate 1 where the common contact hole39 is formed. Herein, the blocking layer 41 is formed such that itcovers the first drain region 15 d of the semiconductor substrate 1, thetop surface of the second interlayer insulating layer 37, and apredetermined portion of the second interlayer insulating layer 37disposed at an inlet of the common contact hole 39, but it does notcover the semiconductor single crystalline layer 23 on the sidewalls ofthe contact hole 39. The blocking layer 41, which may, for example,comprise a titanium nitride layer or the like, may be formed using amethod of selectively covering the common contact hole 399, e.g.,sputtering or a physical vapor deposition (PVD).

Referring to FIG. 11, a first metal layer 43 is conformally formed onthe entire surface of the semiconductor device 1. The first metal layer43 may be formed using, for example, atomic layer deposition (ALD) orchemical vapor deposition (CVD). The first metal layer, for example, maybe formed of at least one of titanium, cobalt, nickel, and/or tungsten.After forming the first metal layer 43, an annealing process isperformed to form a first ohmic layer 45 between the first metal layer43 and the semiconductor single crystalline layer 23. Alternatively, thefirst ohmic layer 45 may be formed at the same time with the formationof the first metal layer 43 using the ALD or the CVD process. The firstohmic layer 45, for example, may be formed of a metal silicide, whereinthe metal may be at least one of titanium, cobalt, nickel, and/ortungsten. At this time, on the bottom of the common contact hole 39,there is not formed the first ohmic layer 45 because of the blockinglayer 41.

Referring to FIG. 12, the first metal layer 43, which is not convertedinto the first ohmic layer 45, is removed. Thereafter, the blockinglayer 41 is removed. A second metal layer 47 is conformally formed onthe entire surface of the semiconductor substrate 1. The second metallayer 47 may be formed, for example, using an ALD or CVD process. Thesecond metal layer 47, for example, may be formed of at least one oftitanium, cobalt, nickel and/or tungsten. Afterwards, an annealingprocess is performed again so as to form a second ohmic layer 49 on thesemiconductor substrate 1 exposed by the common contact hole 39. Thesecond ohmic layer 49 may be formed of a metal silicide, wherein themetal may be at least one of titanium, cobalt, nickel, and/or tungsten.The second ohmic layer 49 may be simultaneously formed with thedeposition of the second metal layer 47. A barrier metal layer or adiffusion barrier layer 51 is conformally formed on the entire surfaceof the semiconductor substrate 1. Thereafter, a conductive layer 53 isformed to fill the common contact hole 39. The barrier metal layer 51,for example, may be formed of at least one of titanium nitride layer,tantalum nitride layer, tungsten nitride layer, titanium aluminumnitride layer and/or tantalum aluminum nitride layer. The conductivelayer 53, for example, may be formed of tungsten. A planarizationprocess may be performed on the conductive layer 53 to form the commoncontact plug in the contact hole 39.

In the inverter structure of FIG. 12, the first gate pattern 11, thefirst source region 15 s and the first drain region 15 d, for example,may constitute the first transistor TR1 which has been illustrated inthe equivalent circuit diagram of the conventional inverter withreference to FIG. 1. Likewise, the second gate pattern 33, the secondsource region 36 s and the second drain region 36 d may comprise thesecond transistor TR2 of FIG. 1. The common contact plug may correspondto the drain node C of FIG. 1.

According to embodiments of the present invention, since the ohmiclayers are separately formed on each of the bottom and the sidewall ofthe common contact plug, it may facilitate optimally forming the ohmiclayers to meet requirements or conditions.

Hereinafter, a method of forming an SRAM device having transistorsstacked in three layers according to another embodiment of the presentinvention will be illustrated with reference to FIGS. 13 to 17.

Referring to FIG. 13, a first isolation layer 102 is formed on asemiconductor substrate 100 using, for example, an STI process, or thelike, so as to define an active region. A thermal oxidation process isperformed over the semiconductor substrate 100 to form a first gateoxide layer 104 on the active region. A first gate electrode layer 106and a first capping layer 108 are stacked on the entire surface of thesemiconductor substrate 100 and patterned to thereby form a first drivegate pattern 110 a and a second drive gate pattern 110 b. A first spacer112 is formed to cover sidewalls of each drive gate pattern 110 a and110 b. An ion implantation process is performed to form a first sourceregion 114 s and a first drain region 114 d. Although it is illustratedthat the second drive gate pattern 110 b in FIGS. 13 to 17 is disposedon the isolation layer 102, it is just shown a local cross-sectionalview for the sake of illustrative convenience. Although not shown in thedrawings, the second drive gate pattern 110 b may have a gate oxidelayer and source/drain regions, wherein the source/drain regions may bedisposed on both sides of the second drive gate pattern 110 b. A firstinterlayer insulating layer 116 is formed to cover the drive gatepatterns 110 a and 110 b. The first interlayer insulating layer 116 ispatterned to form a first contact hole 118 which exposes the first drainregion 114 d. Afterwards, a first epitaxial contact plug 120 is formedto fill the first contact hole 118 through, for example, an SEG process.

Referring to FIG. 13, a first semiconductor single crystalline layer 122is formed on the entire surface of the semiconductor substrate 100. Asecond isolation layer 124 is formed on the first semiconductor singlecrystalline layer 122 using, for example, a typical STI process, or thelike. A first load gate pattern 132 a, a second load gate pattern 132 band a second spacer 134 covering sidewalls of each load gate pattern 132a and 132 b are formed, wherein each of the first and second load gatepatterns 132 a and 132 b is provided with a second gate oxide layer 126(not shown in FIG. 13 for second load gate pattern 132 b), a second gateelectrode 128 and a second capping layer 130. Afterwards, an ionimplantation process is performed so as to form a second source region135 s and a second drain region 135 d. Thereafter, a second interlayerinsulating layer 136 is formed and it is patterned to form a secondcontact hole 138 which exposes the first epitaxial contact plug 120,wherein the second contact hole 138 is overlapped with the first contacthole 118. A second epitaxial contact plug 140 is formed to fill thesecond contact hole 138 through the SEG process. A second semiconductorsingle crystalline layer 142 is formed on the second interlayerinsulating layer 136 and the second epitaxial contact plug 140.

Referring to FIG. 14, a third isolation layer 144 is formed on thesecond semiconductor single crystalline layer 142 using, for example,the general STI process, or the like. A transfer gate pattern 152 and athird spacer 151 are formed, wherein the transfer gate pattern 152 isconfigured with a third gate oxide layer 146, a third gate electrode 148and a third capping layer 150. An ion implantation process is performedto form third source/drain regions 153. Thereafter, a third interlayerinsulating layer 154 is formed. Although not shown in the drawings,another transfer gate pattern may be provided on the secondsemiconductor single crystalline layer 142 such that another transfergate pattern is disposed over the second load gate pattern 132 b.

Referring to FIG. 15, the third interlayer insulating layer 154, thesecond semiconductor single crystalline layer 142, the second interlayerinsulating layer 136, the first semiconductor single crystalline layer122 and the first interlayer insulating layer 116 are patterned insequence to thereby form a common contact hole 156 having a widthgreater than that of the second contact hole 138, wherein the secondcontact hole 138 is overlapped with the common contact hole 156. Whenforming the common contact hole 156, portions of the isolation layers102, 124 and 144, a portion of the first capping layer 108 and the firstspacer 112 on one sidewall of the second drive gate pattern 110 b, and aportion of the second capping layer 130 and the second spacer 134 on onesidewall of the second drive gate pattern 110 b, may be removed so thatthe first and second gate electrodes 106 and 128 are exposed.Afterwards, a blocking layer 158 may be formed on the entire surface ofthe semiconductor substrate 1 where the common contact hole 156 isformed. Herein, the blocking layer 158 is formed such that it covers thefirst drain region 114 d of the semiconductor substrate 100, the topsurface of the third interlayer insulating layer 154, and apredetermined portion of the third interlayer insulating layer 154disposed at an inlet of the common contact hole 156, but it does notcover the semiconductor single crystalline layers 142 and 122 onsidewalls of the contact hole 156. The blocking layer 158 (such as atitanium nitride layer or the like) may be formed using a method ofselectively covering the common contact hole 156, e.g., sputtering or aPVD process.

Referring to FIG. 16, a first metal layer 160 is conformally formed onthe entire surface of the semiconductor substrate 100. The first metallayer 160 may be formed using, for example, an ALD or CVD process. Thefirst metal layer 160, for example, may be formed of at least one oftitanium, cobalt, nickel, and tungsten. After forming the first metallayer 160, an annealing process is performed to form a first ohmic layer162 between the first metal layer 160 and the semiconductor singlecrystals 122 and 142, and between the first metal layer 160 and the gateelectrodes 106 and 128, respectively.

Alternatively, the first ohmic layer 162 may be formed at the same timewith the deposition of the first metal layer 160 using the ALD or theCVD process. The first ohmic layer 162, for example, may be formed of ametal silicide, wherein the metal may be at least one of titanium,cobalt, nickel, and/or tungsten. At this time, on the bottom of thecommon contact hole 156, there is not formed the first ohmic layer 162because of the blocking layer 158.

Referring to FIG. 17, the first metal layer 160, which is not convertedinto the first ohmic layer 162, is removed. Thereafter, the blockinglayer 158 is removed. Herein, the first metal layer 160 and the firstohmic layer 162 may be selectively removed using wet etching process. Asecond metal layer 164 is conformally formed on the entire surface ofthe semiconductor substrate 100. Afterwards, an annealing process isperformed again so as to form a second ohmic layer 166 on thesemiconductor substrate 100 exposed by the common contact hole 156. Thesecond ohmic layer 166 may be formed, for example, of a metal silicide,wherein the metal may be at least one of titanium, cobalt, nickel,and/or tungsten. The second ohmic layer 166 may be simultaneously formedwith the deposition of the second metal layer 164. A barrier metal layeror a diffusion barrier layer 168 is conformally formed on the entiresurface of the semiconductor substrate 100. Thereafter, a conductivelayer 170 is formed to fill the common contact hole 156. The barriermetal layer 168, for example, may be formed of at least one of titaniumnitride layer, tantalum nitride layer, tungsten nitride layer, titaniumaluminum nitride layer and/or tantalum aluminum nitride layer. Aplanarization process is performed over the conductive layer 170 to forma common contact plug fill the common contact hole 156.

In the SRAM device of FIG. 17, the first drive gate pattern 110 a maycorrespond to one of the gate patterns of the drive transistors TR1 andTR4 in FIG. 2, and the second drive gate pattern 110 b may correspond tothe other one of the drive transistors TR1 and TR4 in FIG. 2. The firstload gate pattern 132 a may correspond to one of the gate patterns ofthe load transistors TR2 and TR5 in FIG. 2, and the second load gatepattern 132 b may correspond to the other one of the gate patterns ofthe load transistors TR2 and TR5 in FIG. 2. The transfer gate pattern152 may correspond to one of the transfer transistors TR3 and TR6. Thecommon contact plug may be one of the common terminals C1 and C2 in FIG.2.

As stated above, in accordance with the method of forming thesemiconductor device having stacked transistors according to embodimentsof the present invention, since the ohmic layer on the bottom of thecommon contact hole is separately formed from the ohmic layer on thesidewall of the common contact hole, it may be possible to optimallyform the respective ohmic layers to meet requirements or conditions.Accordingly, the contact resistance of the common contact may beminimized so that it is possible to enhance the speed of thesemiconductor device.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1-4. (canceled)
 5. A method of forming a semiconductor device, themethod comprising: forming a lower transistor on a semiconductorsubstrate; forming a lower interlayer insulating layer on the lowertransistor; forming an upper transistor and an upper interlayerinsulating layer covering the upper transistor, on the lower interlayerinsulating layer; forming a common contact hole to expose source/drainregions of the lower transistor by patterning the upper interlayerinsulating layer, source/drain regions of the upper transistor, and thelower interlayer insulating layer; forming a blocking layer to cover thebottom of the common contact hole but to expose the source/drain regionsof the upper transistor; forming a first ohmic layer on the exposedsource/drain regions of the upper transistor; exposing the source/drainregions of the lower transistor by removing the blocking layer disposedon the bottom of the common contact hole; forming a second ohmic layeron the exposed source/drain regions of the lower transistor; and forminga common contact plug in the common contact hole.
 6. The method of claim5, wherein the blocking layer is formed using PVD or sputtering.
 7. Themethod of claim 6, wherein the blocking layer comprises titanium nitridelayer.
 8. The method of claim 5, further comprising forming a barriermetal layer before forming the common contact plug.
 9. The method ofclaim 5, wherein each of the first and second ohmic layers comprises ametal silicide, the metal being at least one selected from the groupconsisting of titanium, cobalt, nickel, and/or tungsten.
 10. The methodof claim 5, further comprising, before the forming the upper transistorand the upper interlayer insulating layer, forming an intermediatetransistor and an intermediate interlayer insulating layer covering theintermediate transistor, on the lower interlayer insulating layer,wherein the intermediate interlayer insulating layer and source/drainregions of the intermediate transistor are patterned when forming thecommon contact hole.
 11. The method of claim 10, wherein the commoncontact hole exposes at least one gate electrode of the lower transistorand the intermediate transistor, and the first ohmic layer is formed onthe sidewall of the exposed gate electrode.
 12. A method of forming asemiconductor device, the method comprising: forming a first transistorincluding a first gate pattern and a first source/drain region on bothsides of the first gate pattern, over a semiconductor substrate, whereinthe first source/drain region is disposed in the semiconductorsubstrate; forming a first interlayer insulating layer to cover thefirst transistor; forming a first epitaxial contact plug in contact withthe first source/drain region through the first interlayer insulatinglayer; forming a first semiconductor single crystalline layer on thefirst interlayer insulating layer, wherein the first semiconductorsingle crystalline layer is in contact with the first epitaxial contactplug; forming a second transistor including a second gate pattern and asecond source/drain region on both sides of the second gate pattern,over the first semiconductor single crystalline layer, wherein thesecond source/drain region is disposed in the first semiconductor singlecrystalline layer; forming a second interlayer insulating layer to coverthe second transistor; forming a common contact hole to expose thesemiconductor substrate by patterning the second interlayer insulatinglayer, the first semiconductor single crystalline layer, and the firstepitaxial contact plug; forming a blocking layer to cover the bottom ofthe common contact hole but to expose the sidewall of the firstsemiconductor single crystalline layer; forming a first ohmic layer tocover the sidewall of the first semiconductor single crystalline layer;removing the blocking layer; forming a second ohmic layer on thesemiconductor substrate on the bottom of the common contact hole; andforming a common contact plug to fill the common contact hole.
 13. Themethod of claim 12, wherein the blocking layer is formed using PVD orsputtering.
 14. The method of claim 13, wherein the blocking layercomprises titanium nitride.
 15. The method of claim 12, furthercomprising forming a barrier metal layer before forming the commoncontact plug.
 16. The method of claim 12, wherein each of the first andsecond ohmic layers is formed of a metal silicide, the metal being atleast one selected from the group consisting of titanium, cobalt,nickel, and/or tungsten.
 17. The method of claim 12, wherein the formingof the first semiconductor single crystalline layer comprises: formingan amorphous semiconductor layer on the semiconductor substrate wherethe first epitaxial contact plug is formed; and transforming anamorphous structure of the amorphous semiconductor layer into a singlecrystalline structure by performing an annealing process.
 18. The methodof claim 12, further comprising, before the forming of the commoncontact hole: forming a second epitaxial contact plug in contact withthe first semiconductor single crystalline layer through the secondinterlayer insulating layer; forming a second semiconductor singlecrystalline layer on the second interlayer insulating layer, wherein thesecond semiconductor single crystalline layer is in contact with thesecond epitaxial contact plug; forming a third transistor including athird gate pattern and a third source/drain region on both sides of thethird gate pattern, over the second semiconductor single crystallinelayer, wherein the third source/drain region is disposed in the secondsemiconductor single crystalline layer; and forming a third interlayerinsulating layer to cover the third transistor, wherein the thirdinterlayer insulating layer and the second semiconductor singlecrystalline layer are also patterned when forming the common contacthole, and the first ohmic layer is also formed on the sidewall of thesecond semiconductor single crystalline layer.
 19. The method of claim18, wherein the forming of the second semiconductor single crystallinelayer comprises: forming an amorphous semiconductor layer on thesemiconductor substrate where the second epitaxial contact plug isformed; and transforming an amorphous structure of the amorphoussemiconductor layer into a single crystalline structure by performing anannealing process.
 20. The method of claim 12, wherein the commoncontact hole exposes at least one of the first and second gate patterns,the first ohmic layer being formed on the sidewall of the exposed gatepattern.